1. Field of the Invention
This invention relates to a carrier tape used for assembling semiconductors.
2. Description of Related Art
Conventional carrier tape films include those of two kinds i.e. those of the so-called three layer structure as illustrated in steps (a)-(d) of FIG. 1, obtained by coating an adhesive 2 onto a polyimide film 1, followed by punching sprocket holes 3 and a device hole 4, contact-bonding a copper foil 5 onto the film on heating and forming a lead 6 according to photolithography technique, and those of the so-called two layer structure as illustrated in steps (b)-(d), of FIG. 2, obtained by coating and baking a polyimide varnish 1 onto a copper foil 5, followed by forming a device hole 4 by etching on the polyimide surface 1 according to lithography technique and forming a lead 6 on the copper surface again according to lithography technique.
However, according to the three layer structure as illustrated in FIG. 1, the heat resistance of the adhesive layer ((2) in FIG. 1) placed between the polymide film and the copper foil is inferior to that of the polyimide film; hence deviation of the lead occurs due to heating at the time of inner lead bonding for thermally contact-bonding the lead to IC tip so that rejects are liable to occur.
In particular, as the high integration of IC tip advances, carrier tapes also advance toward increase in the number of pins; thus as the number of pins increases, conventional wire-bonding mode requires a longer bonding time and there is a strong possibility that bonding becomes impossible depending on the number of pins. Thus, a collective bonding mode (TAB mode), using a film carrier has been noted. However, as increase in the number of pins advances, the width of lead pitch also becomes smaller; thus the stability of the dimensional accuracy at the time of bonding has been becoming very important.
In such a situation, since an adhesive layer having a limited heat-resistance is present, deviation or torsion of lead occurs due to deterioration or softening of the adhesive brought about by heat at the time of inner lead bonding so that positioning at the time of bonding becomes difficult or inferior bonding is liable to occur.
In addition, there occur cases that impurities contained in the adhesive induce an erroneous motion of IC or a portion of the adhesive adheres to the sprocket holes to cause the positioning of the lead and the IC tip to get out of order.
On the other hand, there is a carrier tape of a two layer structure without any adhesive layer, having overcome the drawback of the above carrier tape of a three layer structure. As illustrated in (b) to (d) of FIG. 2, the carrier tape of the structure is prepared by coating and baking a polyimide varnish onto a continuour copper foil to form a copper-lined tape of a two layer structure, followed by coating a resist onto the polyimide surface, subjecting the resulting material to light exposure development treatment by the medium of a mask and etching the polyimide film to form a device hole. However, the etching rate of the polyimide film is so low that the product cannot be applied only to a carrier tape of a thin polyimide film. In the case of such a thin polyimide film, the stiffness of the resulting tape is so low that troubles are liable to occur at the subsequent steps requiring stiffness such as etching or plating step of copper foil, and further inner lead bonding step. Further, since the photolithography is added, the steps are so long that the product is economically inferior.
Further, there is a process of forming a thin copper film on a polyimide film by means of an electronic beam deposition, a spatter or a non-electrolysis and electrolysis, but the resulting copper film formed according to the process has an inferior flexibility; hence when a fine lead is formed, the product is liable to be broken when it is handled. In addition, the process is inferior in productivity and economically inferior in the aspect of cost.